
Posted: 11/15/2012 7:21:11 PM EST
[Last Edit: 11/16/2012 3:14:13 AM EST by Skibane]

Originally Posted By corigan:
I want to install solar panels and a battery bank in my home to run a 12,000 BTU window AC, a refrigerator, and some lights.
The window unit is a 120V, 10.2A, 1224Watt unit that would run 24 hours a day. Typically its only pulling about 8 amps.
The fridge is about 20 cubic foot and my meter says it draws about 6.5 amps on 120V.
How many watts worth of panel and what size battery bank would I need to run this 24/7 in case of a power outage? I'm not really knowledgable on this solar voodoo so any help from someone who knows would be greatly appreciated.
It's impossible to answer your question without knowing (1.) how much power each of your anticipated loads draws, and (2.) how long each of them will run. It is unlikely that the compressor on your window unit will run all the time, and your fridge will also cycle on and off. So, without knowing how long each of these devices actually runs, you can't proceed any further.
Using a KillAWatt will help you pin down these figures.
After you know them for each load, you can add them all up, and then use them to determine how big your batteries and solar panels need to be in order to meet that estimate.
So  Step One is to fill in the blanks:
Refrigerator power consumption: _________ watts
Refrigerator hours of runtime per day: ________ hours
Window AC unit power consumption: _________ watts
Window AC unit hours of runtime per day: ________ hours
Misc. lights power consumption: _________ watts
Misc. lights hours of runtime per day: ________ hours
AN EXAMPLE:
FRIDGE: Draws 185 watts, and runs an average of 3.2 hours per day.
185 watts x 3.2 hours = 592 watthours (or 0.592 KWH) per day.
AC WINDOW UNIT: Draws 530 watts, and runs an average of 9.4 hours per day.
530 watts x 9.4 hours = 4,982 watthours (or 4.982 KWH) per day.
LED LIGHTS: Draw 12 watts, and run an average of 5.6 hours per day.
12 watts x 5.6 hours = 67 watthours (or 0.067 KWH) per day.
Step 2 is to add them all up:
0.592 KWH + 4.982 KWH + 0.067 KWH = 5.641 KWH per day
So, your batteries and solar panels will need to supply at least 5.641 KWH per day, in order to keep up with your energy consumption.
Step 3 is to calculate the required size of your solar panels:
Assuming 5 hours of full sunshine per day, a 45 watt panel is capable of producing
45 watts x 5 hours = 225 watthours (or 0.225 KWH) per day.
So, to meet your 5.641 KWH per day requirement, you would need:
5.641 KWH per day / 0.225 KWH per day per panel = approx. 25 panels.
You will also need a solar panel charge controller and connecting wires capable of handling this many solar panels.
Step 4 is to calculate the required size of your batteries:
A pair of 6 volt golf cart batteries (connected in series to produce 12 volts) is capable of storing around 220 amphours of energy:
220 amphours x 12 volts = 2,640 watthours (or 2.64 KWH).
So, in order to store enough energy to meet your 5.641 KWH per day requirement, you would need:
5.641 KWH / 2.640 KWH per battery pair = 2.13 battery pairs.
In the real world, you would buy at least 3 battery pairs (6 golf cart batteries).
Note that this estimate assumes you get a full 5 hours of sunshine every day. If you want your system to supply the same 5.641 KWH per day even during rainy or overcast weather, you'll need to increase the number of batteries and solar panels accordingly.
Typically, battery banks are sized to provide at least several days of power on their own, in order to handle overcast or rainy days. Also, the lifespan of the batteries is drastically shortened if they're heavily discharged on a regular basis  so it pays to "go large" on your battery bank size. For the example mentioned above, doubling or tripling the number of batteries would be prudent  i.e., 1218 golf cart batteries.

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